1. Field of the Invention
The present invention relates to work vehicles, and more particularly to a drive control system for a work vehicle provided with a power transmission device having a drive mode switching mechanism that performs drive mode switching between a four-wheel drive mode in which drive power is transmitted to front and rear wheels, and a two-wheel drive mode in which drive power is transmitted to only the rear wheels.
2. Description of the Related Art
A drive control system for a work vehicle that automatically switches the work vehicle from the two-wheel drive mode to the four-wheel drive mode depending on the situation is known from JP 10-114229A.
With the vehicle disclosed in JP 10-114229A, as a result of the drive mode being automatically switched from the two-wheel drive mode to the four-wheel drive mode when a slip ratio calculated from the rotation speed of the left and right rear wheels is greater than or equal to a predetermined value, slippage of the vehicle is eliminated (the slip ratio is improved) and drive power (traction) on the contact area of the vehicle is increased.
However, with this work vehicle, the drive mode is first switched from the two-wheel drive mode to the four-wheel drive mode when a certain degree of slippage has occurred, making it difficult to increase the drive power on the contact area quickly.
Furthermore, because the drive control system does not take the inclination of the body of the work vehicle (inclination of the contact area) into consideration, problems such as the following occur.
For example, assume that slippage occurs when the work vehicle is going up a slope in the two-wheel drive mode (the work vehicle is inclined). In this case, the work vehicle is automatically driven in the four-wheel drive mode when the slip ratio is greater than or equal to a predetermined value, enabling the slippage to be eliminated. Once the slippage is eliminated, the work vehicle again returns to the two-wheel drive mode. However, since the work vehicle is still going up the slope, the work vehicle again slips and is automatically switched to the four-wheel drive mode. In other words, in the case where slippage occurs when the work vehicle is inclined, a situation could possibly arise where the work vehicle is endlessly switched alternately between the two-wheel drive mode and the four-wheel drive mode (hunting state).
In view of this situation, there is also demand for a drive control system for a work vehicle that is able to appropriately eliminate slippage even when the work vehicle is inclined and/or a drive control system in which drive power is quickly increased under circumstances where it is desirable to increase drive power on the contact area of the work vehicle.